The present invention relates to wideband transmitters, but more specifically, to a method and an apparatus to produce spectrally-controlled wideband pulses with high DC-to-RF power efficiency that may be used in communications, radar or geopositioning systems.
The need to spectrally control emissions of a UWB signal at high DC-to-RF power efficiency is important for both regulatory and practical reasons. From a regulatory perspective, the ability to generate ultra wideband, or short pulse, waveforms that avoid operation in select bands is often necessary to prevent interference with existing radio-frequency (RF) services. Furthermore, in many cases, a flat power spectral density of a wideband emission in which each unit Hertz of bandwidth contains essentially the same amount of energy, is desirable to minimize interference to narrowband receivers operating over the same band. Thus, spectral shaping, filtering, or band-limiting—for both frequency translation and spectral whitening—is usually desirable in UWB transmitter designs. From a practical perspective, a high DC-to-RF efficient pulse transmission also permits extended operational life of the equipment in battery-operated designs.
Larrick et al. (commonly-owned U.S. Pat. No. 6,026,125) discuss methods for generating spectrally controllable UWB transmissions. There, a low level impulse excitation is spectrally shaped or filtered, and subsequently heterodyned (if necessary) to reach a desired operational center frequency. Subsequent time-gated linear amplification is then used to achieve the desired peak power output. In Larrick et al., the impulse generator is physically distinct from the filter which, in turn, is physically distinct from the gated output amplifier. As Larrick et al. point out, impulse excitation is typically generated at a low RF level to enable faster switching (e.g., higher data rates of operation) and to prevent overload of any subsequent amplifier stages that are operated either in their linear mode or near output saturation levels. To reach a desired peak power output, subsequent gated power amplifier stages may be added. This, however, adds further cost and complexity to the system.
More rigorously, the impulse generator of Larrick et al. can be treated as a Thevenin source consisting of an equivalent “ideal” impulse generator and an output impedance that is typically impedance-matched with following filter circuitry. In conventional designs, such impedance-matching was achieved (particularly because of the wide range of frequencies over which the match must be realized) through the use of passive, lossy elements such as resistive pads or attenuators.
The subsequent shaping or bandpass filter network connected to the low level impulse source was thus doubly terminated; having its input terminated by the equivalent Thevenin impedance of the impulse generator and having its output terminated by the input impedance of the subsequent power amplifier stages. Energy efficiency was obtained by time-gating the power amplifier so that it was active only for a time period roughly equivalent to the time of occurrence and duration of the filtered UWB pulse.
However, additional time had to be allowed for inevitable transient effects associated with gated switching of the power amplifier so that, in practice, the power amplifier was active for a substantially longer time than the duration of the filtered UWB pulse. Furthermore, such power amplifier devices were typically operated in their linear mode during pulse production; and, hence, were ultimately limited in their DC-to-RF conversion efficiency, even in the gated regime.